Effects of human milk odor stimulation on feeding in premature infants: a systematic review and meta-analysis

Previous studies suggested odor stimulation may influence feeding of premature neonates. Therefore, this systematic review and meta-analysis of randomized controlled trials was conducted to assess the effect of human milk odor stimulation on feeding of premature infants. All randomized controlled trials related to human milk odor stimulation on feeding in premature infants published in PubMed, Cochrane, Library, Medline, Embase, Web of science databases and Chinese biomedical literature databases, China National Knowledge Infrastructure, China Science and Technology Journal Database (VIP) and Wanfang Chinese databases were searched, and The Cochrane Handbook 5.1.0 was used to evaluate the quality and authenticity of the literature. Relevant information of the included studies was extracted and summarized, and the evaluation indexes were analyzed using ReviewManager5.3. The retrieval time was from the establishment of the database to July 28, 2022.12 articles were assessed for eligibility, and six randomized controlled studies were eventually included in the meta-analysis (PRISMA). A total of 6 randomized controlled studies with 763 patients were finally included in the study, and the quality evaluation of literatures were all grade B. Human milk odor stimulation reduced the transition time to oral feeding in premature infants [SMD = − 0.48, 95% CI (− 0.69, − 0.27), Z = 4.54, P < 0.00001] and shortened the duration of parenteral nutrition [MD = − 1.01, 95% CI (− 1.70, − 0.32), Z = 2.88, P = 0.004]. However, it did not change the length of hospitalization for premature infants [MD = − 0.03, 95% CI (− 0.41, 0.35), Z = 0.17, P = 0.86]. The implementation of human milk odor stimulation can reduce the transition time to oral feeding and the duration of parenteral nutrition in premature infants, but further studies are needed to determine whether it can reduce the length of hospital stay in premature infants. More high-quality, large-sample studies are needed to investigate the effect of human milk odor stimulation on the feeding process and other outcomes in premature infants.

nursing Premature infants are babies born alive before 37 weeks of pregnancy are completed.According to the relevant World Health Organization (WHO) report, 15 million premature infants are born every year in the world 1 .Preterm birth is an important public health issue, as it is associated to a high burden of mortality and morbidities 2 .Premature infants are at high risk for aspiration due to poor coordination of sucking and swallowing 3 .Thus, they usually need tube feeding for nutritional needs with a gradual transition to oral feeding.Premature infants need to start oral feeding at the youngest possible age to improve survival and recovery 4 .
It has been shown [5][6][7] that fetal olfactory receptors begin to appear in the 8th week of pregnancy, ciliated olfactory receptors mature in the 24th week, and the nasopharyngeal epithelium can express olfactory marker proteins in the 28th week.Premature infants, just like full-term infants, possess a more advanced olfactory system at birth, enabling them to detect, selectively process, retain, and recall odor information.They are able to distinguish

Inclusion criteria
Study characteristics used as criteria for eligibility are as follows: (1) Premature infants born at less than 37 weeks gestation who are receiving tube feeding and/or parenteral nutrition; (2) randomized controlled trials; (3) both groups of premature infants were given tube feeding and/or parenteral nutrition, with interventions involving the application of human milk odor stimulation in the intervention group and routine care in the control group; (4) evaluation metrics included transition time to oral feeding, length of stay, duration of parenteral nutrition, and/or body weight; (5) English or Chinese.

Exclusion criteria
(1) Duplicate articles; (2) Preclinical study, meta-analysis, case reports, reviews, guidelines; (3) Valid ending data unable to be extracted or calculated; (4) Full text of the study is not available; (5) The quality evaluation is grade C.

Data extraction
Two authors (YW and AP) carried out the data extraction process independently.Any disagreement was resolved with a senior researcher (CS) through discussion and consensus.Extracted contents were listed as follows: (1) Basic information of the included articles (title, the first author's name, year of publication, geographic locations, the quality of the studies).( 2

Quality assessment
The quality of the selected studies was evaluated by two investigators using a revised tool for assessing risk of bias in Review Manager software.According to the Cochrane intervention research system evaluation manual 5.1.0,the document authenticity evaluation standard is carried out 16 .It mainly includes five aspects of bias (selection bias, performance bias, detection bias, attrition bias, reporting bias), six evaluation items: the generation method of random sequence, the concealment of random scheme allocation, the blind method of subjects and interventions, the blind method of outcome evaluators, the integrity of outcome data (loss of follow-up), and the possibility of selective reporting of research results.The single evaluation item is divided into three grades: (1) "low risk of bias" when a low risk of bias was determined for all domains, (2) "high risk of bias" when high risk of bias was reached for at least one domain or the study judgment included some concerns in multiple domains, and (3) unclear risk of bias 17 .The final quality evaluation grades of the literature are Grade A, grade B and grade C.

Statistical analysis
The main statistical software used in this study was ReviewManager5.3;Cochrane library) software.Measures such as length of hospital stay, duration of transitional oral feeding, and duration of parenteral nutrition use were statistically analyzed using the mean ± standard deviation and 95% CI.Standardized conversions could be performed with different measurement instruments to calculate MD/SMD values and 95% CI.The heterogeneity of included studies was examined by the I 2 index.If the test showed a high level of heterogeneity (I 2 > 50%), a random effect model was used, otherwise a fixed-effect model (I 2 < 50%) was used 18 .Sensitivity analysis was also performed to investigate the potential interference to the pooled effect size 19 .Statistical significance was set at P < 0 0.05.

Ethics approval and consent to participate
This is a systematic review, no ethics review.

Literature search results
Initially, 322 literatures related to olfactory stimulation applied to premature infant were searched until July 28, 2022, of which 145 literatures related to human milk odor feeding of premature infants were screened.After excluding duplicate publications and those without full texts, 49 studies remained for full text screening.Reading through the full text, 26 articles were finally retained after excluding the inconsistent literature from the three aspects of study topic, overall design, and evaluation index.Then 12 articles were assessed for eligibility, and six randomized controlled studies were eventually included in the meta-analysis [20][21][22][23][24][25] (Fig. 1).General information and characteristics of the included literature are detailed (Table 1).

Quality assessment of the selected studies
The qualities of the six included literatures were evaluated as Grade B (partially meeting all criteria).Most of the studies failed to demonstrate the concealment of random scheme allocation and the blind method of outcome evaluators (Fig. 2, Table 2).

Effects of human milk odor stimulation on the transition time of oral feeding in premature infants
Five studies [20][21][22][23][24] evaluated the transition time of oral feeding for premature infants.Due to the small heterogeneity among studies (P = 0.39, I 2 = 3%), fixed-effect model analysis was conducted.The result showed that the transition time of oral feeding for premature infants in the intervention group was statistically significantly shorter than that in the routine care group,the statistical unit of the outcomes were days [SMD = − 0.48, 95% CI (− 0.69, − 0.27), Z = 4.54, P < 0.00001] (Fig. 3).www.nature.com/scientificreports/

Effects of human milk odor stimulation on duration of parenteral nutrition in premature infants
Two studies 24,25 evaluated the duration of parenteral nutrition of premature infants.Due to the small heterogeneity among studies (P = 0.43, I 2 = 0%), the fixed effect model was used to analyze the duration of parenteral nutrition.
The result indicated a statistically significantly shorter duration of parenteral nutrition support in the intervention group than that in the routine care group, the statistical unit of the outcomes were days [MD = − 1.01, 95% CI (− 1.70, − 0.32), Z = 2.88, P = 0.004] (Fig. 4).

Effects of human milk odor stimulation on the length of hospital stay for premature infants
Four 20,21,24,25 studies explored the impact of human milk odor stimulation intervention on the length of hospital stay.Random-effect model was applied given the high heterogeneity (P = 0.04, I 2 = 65%), which found no statistically significant difference between the intervention group and routine nursing care group [MD = − 0.28, 95% CI (− 1.19, 0.63), Z = 0.06, P = 0.55] (Fig. 5A).To explore the source of heterogeneity, sensitivity analysis was conducted by omitting one study at a time, which found that the study conducted by Yildiz could be have a significant impact the heterogeneity 20 .After removing this study 20 , heterogeneity was dramatically reduced (P = 0.50, I 2 = 0%), and fixed effect model was used to assess the effect of human milk odor stimulation intervention on the length of hospital stay.The result still showed no statistically significant difference between the two groups, the statistical unit of the outcomes were days [MD = − 0.03, 95% CI (− 0.41, 0.35), Z = 0.17, P = 0.86] (Fig. 5B).

Discussion
The systematic review and meta-analysis of six randomized controlled studies found improved outcomes of premature infants associated with human milk odor stimulation.Premature infants are difficult to be fed through bottles by mouth due to underdeveloped oral motor function and uncoordinated sucking, swallowing and respiratory movements, which usually require formula or human milk delivered through a gastric tube 26 .In addition, oral exercise by oral intake contributes to weight gain and neurological development and accelerate their recovery process, while non-oral feeding deprives premature infants of oral exercise [27][28][29] .Moreover, prolonged tube feeding affects the oral motor skills of the child, leading to reduced respiratory coordination, late sensory problems, and malnutrition 30 .Malnutrition leads to lack of stable weight gain, prolonged hospitalization, and even neurological deficits and readmission 31 .In contrast, adequate nutrition, maintenance of weight gain, and physiological stability play crucial roles in the successful recovery of premature infants from hospitalization 22 .Therefore, the transition from parenteral or tube feeding to complete oral feeding will contribute significantly to sufficient nutrition and prompt recovery of premature infants.The results of the pooled analyses in the present study showed that human milk odor stimulation was able to reduce the time required for transition to normal oral feeding in premature infants.It is well known that normal oral feeding (sucking, swallowing and respiratory Intervention group: 1.On the basis of conventional tube feeding, the premature infant was given olfactory stimulation 5 min before tube feeding.2. The collected mother's breast milk was pasteurized and refrigerated for spare, and rewarmed to 40 °C before feeding.3. 5 ml of mother's breast milk drops were pumped in sterile gauze (size: 5 cm × 5 cm) using a syringe, and the nurse wore sterile gloves and then held the gauze about 2 cm above the nostrils of the premature infant for about 2 min Control group: after the gastric tube was placed according to the specialized nursing operation protocol, tube feeding was performed, giving priority to mother's breast milk, and when mother's breast milk was insufficient, special formula for premature infants was used for feeding Intervention frequency and duration: 3 times a day, in the morning, at noon and in the afternoon before tube feeding, and stopped when the gastric tube was removed in premature infants.Subjects in both study groups were followed up until hospital discharge coordination) is an early sign of neuromotor integrity in premature infants and an important indication for hospital discharge 28 .Premature infants admitted to the newborn intensive care unit (NICU) for further treatment and care after birth often require controlled number, frequency, and volume of feedings.Therefore, these newborns may lack adequate stimulation and sensory experiences related to feeding, such as hunger, fullness, taste, and smell 32 .Olfactory and gustatory stimulation alone or in combination can reduce gastrointestinal-related adverse reactions and effectively improve the nutritional status of premature infants by activating complex pathways and triggering cephalic responses, which increase intestinal motility, digestive enzyme secretion, and hormone release [33][34][35] .Therefore, the application of human milk odor stimulation plays a vital role in promoting the recovery process of premature infants.
Although human milk odor stimulation was associated with reduced transition time to oral feeding and short duration of parental feeding, it did not change the length of hospitalization.Hospitalization in premature infants ) Baseline characteristics of the subjects in the eligible literature.(3) Detail of interventions or exposure factors.(4) The outcome indicators and outcome measures of interest (MD and SMD with the corresponding 95% CI).

Figure 5 .
Figure 5. (A) Effects of human milk odor stimulation on the hospitalization time for premature infants before removing Yildiz's study; (B) Effects of human milk odor stimulation on the hospitalization time for premature infants after removing Yildiz's study.